Add facility to generate RPC docs

Summary: This is a modified version of https://github.com/bitcoin-core/bitcoincore.org/tree/master/contrib/doc-gen so that it fits into our build system.

Test Plan:

ninja bitcoind

Run bitcoind as a daemon

ninja doc-rpc

Verify that the RPC doc are generated properly.

ninja clean

Verify that the RPC doc are now gone.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6382